1. Field
This disclosure relates to a method for growing a thin film. More particularly, this disclosure relates to a method for growing a thin film, which includes modifying a surface grain size and surface roughness on a thin film to improve the mobility of a carrier and a light scattering effect.
2. Description of the Related Art
In general, ZnO thin films are used as electrode materials for solar cells or flat panel display devices due to their excellent light transmission characteristics and conductivity. Such ZnO thin films are generally formed via a chemical vapor deposition (CVD) process or a physical vapor deposition (PVD) process such as sputtering. Various vapor deposition processes and physical properties of thin films obtained thereby are disclosed in many literatures.
Electrical and optical properties of ZnO thin films are significantly affected by the microstructures of the thin films. ZnO thin films have conductivity determined by the density and mobility of a carrier. Particularly, the density of a carrier is determined by crystal defects or doping levels, while the mobility of a carrier depends on light scattering that occurs during the movement of the carrier. In the case of a ZnO thin film, one of the most important factors affecting the mobility is to reduce the density at grain boundaries. Therefore, it is very important to reduce the density at grain boundaries by increasing the grain size of the thin film so that the thin film has improved conductivity.
Meanwhile, when a ZnO thin film is used in a thin film type solar cell, light confinement is required to increase a light transmission distance for the absorption of sunlight. To accomplish such light confinement, it is required to induce light scattering by increasing the surface roughness of a ZnO thin film. In the case of an SnO2 thin film, micrometer (μm)-scale surface roughness is naturally obtained by the growth surface of growing grains. In contrast, this cannot be realized in the case of a ZnO thin film, because a ZnO thin film has a very small grain size and shows very low surface roughness as grains grow into (0001) surface. Therefore, ZnO thin films require a chemical etching process to ensure a certain degree of surface roughness. However, side effects may occur due to the chemical etching.
Meanwhile, thin films formed via a CVD or PVD process generally grow while forming a column-like grain structure. Column-like grains forming the column-like grain structure merely have a diameter of several tens nanometers (nm). Therefore, it is necessary to form large-diameter grains to improve conductivity or light confinement in various applications using ZnO.
The diameter of column-like grains increases as the thickness of a thin film increases. However, a ZnO thin film deposited via sputtering does not provide a sufficiently increased diameter of column-like grains even when the thickness of the thin film is increased. Thus, it is not expected in such ZnO films that the grain size is increased significantly. On the contrary, a ZnO thin film deposited via a CVD process provides an increased grain size as the result of an increase in the thickness. Particularly, it is shown that a thin film having a thickness of 8 μm provides a surface grain size of about 1 μm, which is several tens of times greater than the surface grain size of a thin film formed via sputtering. However, ZnO thin films used in solar cells have a thickness of merely about 1-2 μm in general. Therefore, when forming such ZnO thin films via a CVD process, the grain size is at most 1 μm.
In addition to the above-described sputtering process and CVD process, a method for controlling the grain size via surface treatment of a substrate may be considered. The method includes reducing the density of nuclei initially formed on the substrate during the growth of grains to increase the grain size. For example, it is theoretically possible to provide grains that form a thin film after vapor deposition with a surface grain size of 30 μm or more, if a glass substrate is subjected to specific surface treatment by which nuclei are formed with an interval of 30 μm. However, nucleation of ZnO actually occurs at a high rate. Thus, the glass substrate requires surface treatment by which wetting with ZnO is inhibited in order to realize such a large surface grain size. Additionally, it is required to set a condition capable of minimizing driving force of deposition (e.g. supersaturation of deposition gas). However, even when such a condition is set, it results in a very low thin film growth rate. As a result, such surface treatment is not practical.